When a packaging bag, having retort food, frozen food or the like packed therein in a hermetically sealed state, is heated by a microwave oven, the pressure in the interior of the packaging bag increases due to vapor etc. produced from the heated contents, and thus, the packaging bag may burst so that the packed contents scatter and soil the interior of the microwave oven and even inflict harm, such as a burn, on a human body.
In order to avoid the aforementioned inconveniences, it has been conventional to partly open the packaging bag or make a hole in the body of the packaging bag before the packaging bag is subjected to heating by a microwave oven, so as to discharge vapor etc. produced within the bag and thereby prevent the bag from bursting.
However, such a conventional solution would require extra time and labor on the part of general consumers. Also, because the vapor produced due to the heating by the microwave oven is immediately discharged outside the packaging bag, a steaming effect by the vapor would be considerably reduced so that the food undesirably deteriorates in flavor.
To avoid the problem, a variety of plastic pouches have so far been proposed which are equipped with a mechanism that automatically opens, in response to an increase in the interior pressure of the pouch due to heating by a microwave oven, so that the increased interior pressure is allowed to automatically escape from the interior of the pouch.
As the plastic pouches equipped with such an automatically-opening mechanism, there have been known various types of plastic pouches, such as standing-type pouches that are heated in a self-erected position within a microwave oven (see, for example, Japanese Patent Application Laid-open Publication Nos. 2002-249176 and 2003-192042), flatly-laid-type pouches, such as branch-type pouches equipped with an automatically-opening mechanism provided in a flat bag or branch portion of the bag, that are heated in a flatly-laid position within a microwave oven (see, for example, Japanese Patent Application Laid-open Publication Nos. 2002-80072 and 2001-106270, and Japanese Patent Publication No. HEI-8-25583).
Of these pouches, the most superior in terms of productivity and cost is the flat-type pouch. However, because the opening portion of the flat-type pouch can not be held stably at a high position during cooking by the microwave oven and after the pouch automatically opens due to an increase in the interior pressure, the flat-type pouch would present the inconvenience that the contents of the pouch undesirably spout or leak out of the automatically-opening portion.
For this reason, it has heretofore been proposed to employ an auxiliary device, such as an item packaging box, for holding the opening portion of the flat-type microwave-oven-cooking pouch (see Japanese Patent Application Laid-open Publication No. 2003-170930); however, the use of the auxiliary device would require cumbersome operation and lead to an increase in the cost.